1. Field of the Invention
This system and method relate to an optical reading of luminescent barcodes on merchandise and similar items. More specifically, the system and method relates to the improved readability of barcodes on specularly reflecting materials.
2. Description of the Prior Art
Several industries track and maintain information by reading data encoded symbols, such as a barcode. In present day merchandising operations, data pertaining to the purchase of a merchandised item is obtained by reading a barcode printed on the merchandised item. Barcoded information may also be used by an organization internally to track inventory and to maintain documentation. Hospitals, for example, may use barcoded information to track patients and their records. In order to standardize the barcodes used at various points of the read-out system, several industries have adopted various standardized codes. Retail food suppliers have adopted the Uniform Product Code (UPC) in the form of a barcode, while other industries use standardized codes which most accurately meet their needs. Various systems have been constructed to read this barcode, including hand-held wands which are moved across the barcode, and stationery optical reading systems, typically located within the check-out counter of a grocery store. The barcode is read when a purchased merchandised item is passed across a window constituting the scanning area of the counter.
Typically, barcodes are read by illuminating the coded surface and detecting the variations in contrasts of the bars and spaces. The ratio of the width of white bar elements and black bar elements, along with the order in which the bar elements are placed specifies the encoded information. In order to achieve an adequate reading a number of conditions must be met. First, the illumination source must reflect enough light from the coded surface and through the aperture of the decoder to give the decoder the needed signal strength to produce a usable output. Second, the contrast between the bars and spaces of the barcode, as perceived by the detector, must be sufficient to produce signal levels and signal-to-noise ratios large enough to convey the desired information. And third, either the detector or the illumination source must be able to convert the light and dark areas corresponding to the bars and spaces into a serial information stream of sufficient regularity and consistency to allow the decoder to properly interpret the contrast information.
Barcode readers are typically categorized according to the relative aperture size of their illumination source and detector. An aperture is an opening that emits light from a source or admits light into a detector, and is measured according to the diameter of either the opening or the light beam. Scanning helium-neon laser barcode readers use a laser illumination source of small aperture in conjunction with a relatively large area detector. The detector aperture is generally between 100 to 1000 times the area of the light source aperture. Hand-held wands also use a laser illumination source of a small aperture with a large area detector. In the general case of a small source/large detector reader, the reader is dependent upon the reflective characteristics of the coded material to ensure that sufficient radiation is reflected from the barcoded material onto the detector aperture to provide usable signal levels. The reflective characteristics of barcoded material may be specular or diffuse in nature. In theory, specular material reflects a light beam directed perpendicular to the material with no change in the cross-sectional area of the beam. In reality, specular material reflects this beam with generally little change in the beam's cross-sectional area. Diffuse material causes the reflected beam to disperse, increasing the angle of the beam directed to the detector. In the case of highly specular materials, the reflection of the generally collimated illumination source is contained within a narrow angle. Therefore, the angle of the coded surface relative to the incident radiation and the location of the detector must be tightly controlled to ensure that the aperture falls within the reflected light. It is this angular requirement which renders small source/large detector readers marginally usable on specular materials.
A number of systems have been developed to improve the readability of barcoded material, with varying degrees of success. U.S. Pat. No. 3,812,374 to Tuhro (1974) discloses a system for reading a diffuse reflective label. The device suppresses specular reflection commonly used in a system for reading a diffuse reflective barcoded label. The apparatus includes a polarized radiation source having a first polarization and means for providing relative motion between the label and the polarized radiation for scanning a diffuse relative label with the polarized radiation. Polarizing means are provided for selectively blocking specular reflections having the first polarization and passing radiation having other polarization by increasing the signal-to-noise ratio of the diffusively reflected radiation to the specularly reflected radiation.
U.S. Pat. No. 4,013,893 to Hertig (1977) discloses an optical barcode scanning device for high data density barcode readings. The device utilizes a two channel arrangement for directing light reflected from the barcode being read onto two photodetectors in predetermined proportionate amounts. The two channel system detects a barcode edge independent of print contrast variations, resolution modulation or change of illumination, by comparing the optical signal of one channel with the optical signal of the other. In U.S. Pat. No. 4,160,902 to Van Wijngaarden (1979) an optical reading head for reading luminescent barcodes is disclosed. The reading head is provided with a source of light for irradiating the bars of the barcode and a detector which includes a hollow optical guide means for collecting the rays emitted by the bars. To correctly convert the barcodes into electronic signals, it is essential that the bars be very sharply and distinctly observed by the light-sensitive detector.
A holographic system for scanning barcode indicia is disclosed in U.S. Pat. No. 4,333,006 to Gorin, et al. (1982). A laser is directed at a first set of holograms located on a single rotating disc, each hologram generating an individual scan beam having a slightly different focal length and direction angle than that of the other holograms. The generated scanning beams are directed onto a target area and a label, or an object bearing a barcode indicia is passed over this target area. The light reflected from the barcode indicia is picked up by a second set of holograms mounted onto the rotating disk and focused onto an optical detector used to read the barcode.
Several difficulties are encountered in using conventional barcode readers with specularly reflecting materials. The angle needed to correctly read the barcode is unacceptably tight when the barcoded item is held within the range commonly used in barcode systems. Certain combinations of material, illumination source and reading conditions may combine to make convenient barcode reading impossible.